In the past there have been air conditioning systems provided with a gas furnace unit and a heat pump unit, that perform air conditioning in a space to be air-conditioned. Japanese Laid-open Patent Publication No. 64-54160, for example, discloses an air conditioning system provided with a gas furnace unit and a heat pump unit, the air conditioning system being configured so as to select the gas furnace unit as a heat source unit when the outdoor air temperature is less than a predetermined temperature upon initiation of operation, and to select the heat pump unit as a heat source unit when the outdoor air temperature is equal to or greater than a predetermined temperature upon initiation of operation.